This is an anatomical study of the production and differentiation of photoreceptors in the growing, adult goldfish retina. New rods are produced at the periphery, but their proportions increase throughout the retina during growth. The experiments are designed to solve this apparent paradox. 1. A quantitative, ultrastructural assessment of the state of differentiation of the rod population in peripheral and central retina. 2. A categorization and quantification of cells in the inner nuclear layer, to reveal a pool of rod precursors which is selectively depleted with growth. 3. An analysis of the source and fate of new rods using 3H-thymidine radioautography, including a determination of pulse time. 4. An ultrastructural description of the outer plexiform layer, to find evidence of synaptic plasticity.